Host applications being executed by computing devices can provide a mechanism for client applications to execute code and provide content to users. Host applications can provide virtual machines for executing client applications. Virtual machines can provide scripting interactivity at run-time, and support for video, audio, and graphics. Host applications can provide windows within visual displays, such as computer monitors. A host application can allow a client application to display content, such as video, graphics, and text, to users. The client application is also provided with an event stream for receiving input from a user in the form of keyboard key strokes, mouse clicks and movements, touch screen or touch pad input, or other input.